The invention relates to an ultrasound measuring device for a container and a method for determining a useful signal from an ultrasound signal having an interfering signal superimposed on it.
Closed gas bottles having a liquid content have numerous applications in industry and everyday life such as as fuel gas bottles for forklift trucks or as gas bottles supplying energy for camping. Ultrasound measurements for determining the filling level of the gas bottles permit simple and cost-efficient maintenance. A filling level measuring apparatus operating with ultrasound is known, for example, from EP-A-0955529. The apparatus permits filling level control for fuel gas-driven forklift trucks by ultrasound measurement. Therefore, this allows any unnecessary replacement of partially emptied bottles of forklift trucks or breakdown due to empty fuel gas bottles to be prevented.
Published subsequently, DE-A-103 31 044 concerns the support device for a container and for a filling level measuring facility for measuring the filling level of a medium that is present in the container. The coupling/holding device comprises a container receptacle and a locking facility that can be switched between at least one open position, in which the container is freely mobile, and one closed position, in which the container is locked in the container receptacle. In this context, the filling level measuring facility, being in the closed position, can be pressed against an external wall of the container and can be moved away from the container in the open position. No additional action by a user is required for this purpose, i.e. the pressing against and the removal proceed automatically upon opening and closing of the locking facility.
In ultrasound measuring devices of this type, an ultrasound transducer is used to inject an ultrasound signal from below through the container wall into the medium. Once the ultrasound signal travels through the medium up to a filling level, it is reflected at a phase boundary and returns to the ultrasound transducer, whereby the phase boundary may separate, for example, the liquid/gaseous phases of a liquefied fuel gas serving as medium. The filling level can be determined from the travel time of the ultrasound signal that is reflected and received by the ultrasound transducer. However, as an undesired side effect of this ultrasound measurement, transverse waves occur upon the injection of the ultrasound into the container wall and propagate in the container wall. In terms of physics, these transverse waves are related to the elasticity of shape of the container wall that is provided in the form of a solid body. Depending on the features of the container such as geometry, wall thickness, wall structure including varnishing as well as the selected frequency of the ultrasound signal, these transverse waves occur at various strength and lead to an interfering signal that is received by the ultrasound transducer jointly with the useful signal that is reflected at the liquid/gaseous phase boundary and is suitable for determining the filling level. If the interfering signal and the useful signal are superimposed or if the interfering signal is within a possible travel time range of the useful signal serving for determining the filling level, separation of the interfering signal from the useful signal is not possible without further effort, which renders the filling level measurement considerably more difficult.
The invention is based on the object to provide an ultrasound measuring device for a container, in which the interfering signal that is elicited by the container wall in an ultrasound measurement returns to the site of injection of the ultrasound transducer either not at all or clearly attenuated and/or delayed in time as compared to a useful signal. It is another object of the invention to provide a method that can be used to determine the useful signal from an ultrasound signal on which the interfering signal is superimposed.
These objects are met according to the invention by an ultrasound measuring device according to claim 1 and by a method according to claim 12 for determining a useful signal from a ultrasound signal on which an interfering signal is superimposed. Preferred embodiments are evident from the dependent claims and/or are illustrated in the following description.